Reflective liquid crystal displays have come into widespread use in portable applications due to their low power consumption. In particular, reflective liquid crystal display devices capable of full-color display are now under development.
A reflective liquid crystal display device uses ambient light as a light source and can provide display with high picture quality when used outside in sunlight, in contrast, the display of a transmissive liquid crystal display device generally suffers from low contrast and poor visibility when used outside in sunlight. However, the visibility of a reflective liquid crystal display device is poor when used outside at night or in a room with poor lighting and cannot obtain the image quality that is possible in a transmissive liquid crystal display device.
A liquid crystal display device is therefore desired that is capable of reflective display in a bright setting and that can display using a supplementary light source in a dark setting. A semi-transmissive liquid crystal display device can be offered as one example. Such a semi-transmissive liquid crystal display device employs a reflecting surface that reflects incident light as a semi-transmissive surface. However, a semi-transmissive surface having, for example, transmittance of 50% and reflectance of 50% has the disadvantage that display is dark during both reflective display and transmissive display.
A liquid crystal display device that can be switched between reflective and transmissive display is disclosed in Japanese Patent Laid-open No. 119026/99 as a device for overcoming the above-described disadvantage. FIG. 13 shows a schematic side sectional view of a liquid crystal display device that includes a holographic reflecting layer. As shown in FIG. 13, holographic reflecting layer 528, which is a volume hologram, is arranged between backlight unit 503 and liquid crystal layer 501.
This volume hologram is a structure having fixed refractive index modulation, and when the period of this modulation is at the level of visible wavelengths, produces diffraction known as Bragg reflection, whereby light of a particular angle of incidence and a particular wavelength is strongly reflected in a specific direction. This hologram function and the characteristics of a liquid crystal display device having a prior-art holographic reflecting layer can be explained using the circular charts shown in FIG. 14. The radius of the circle shown in FIG. 14(a) is given as n/λ. Here, λ is the wavelength of incident light, and n is the average refractive index of the hologram medium. The incident light is represented as a vector from the center to the circumference of the circle. Incident wave number vector 529 and reciprocal vector 531 are arranged as in FIG. 14(a). The direction of reciprocal vector 531 is defined as the modulation direction of the refractive modulation, and the magnitude of the vector is defined as the inverse of the modulation period. In FIG. 14(a), emitted wave number vector 530 is shown as the difference between incident wave number vector 529 and reciprocal vector 531. Light is strongly emitted in the direction of this emitted wave number vector 530.
Explanation next regards the reflective action using FIG. 14(b). During reflective display, display is realized by only incident light, and backlight unit 503 is not used. Incident light from the surroundings is incident to liquid crystal layer 501 and then incident to holographic reflecting layer 528. If the reciprocal vector of holographic reflecting layer 528 is arranged as shown in FIG. 14(b), only a specific wavelength is selectively reflected in a specific emission direction. The light that is selectively reflected again passes through liquid crystal layer 501 to enable reflective display. Light of wavelengths other than this selectively reflected wavelength is transmitted by holographic reflecting layer 528 and therefore does not contribute to display.
When the surroundings are dark, on the other hand, the backlight is lit up. In this case, the light from the backlight is transmitted by holographic reflecting layer 528 and is incident to liquid crystal layer 501, whereby transmissive display can be realized as shown in FIG. 14(c).
Using the liquid crystal display device that is disclosed in Japanese Patent Laid-open No. 119026/1999 as described above enables display that switches between reflective and transmissive display.
Nevertheless, the liquid crystal display device that is disclosed in the above-described Japanese Patent Laid-open No. 119026/1999 has the disadvantage that the displayed colors during reflective display differ from the displayed colors during transmissive display. More specifically, as shown in FIG. 14(b), only light of a specific wavelength that is determined by the hologram is reflected during reflective display. And as a result, only monochromatic display is possible during reflective display. Further, as shown in FIG. 14(c), during transmissive display, specific wavelengths of the light from the backlight undergo Bragg reflection by the hologram and cannot contribute to transmissive display. The wavelengths that are subject to this Bragg reflection are identical to the wavelength of monochromatic light during reflective display. Thus, even though a color such as green can be displayed during reflective display, the complementary color of green must be displayed during transmissive display. For similar reasons, full-color display is not possible.
Thus, although the liquid crystal display device that is disclosed in Japanese Patent Laid-open No. 119026/1999 realizes display that can be switched between reflective and transmissive display, such a display has the disadvantages that color display is limited and full-color display is not possible.
It is therefore an object of the present invention to provide a liquid crystal display device that employs Bragg reflection and that can realize full-color reflective display in bright surroundings and full-color transmissive display in dark surroundings.